Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A touch-sensitive apparatus comprises a light transmissive panel, an
illumination arrangement and a detection arrangement. The illumination
arrangement is configured to couple light into the panel via an
incoupling site such that the light propagates by total internal
reflection in opposite top and bottom surfaces of the panel and such that
an object touching the top or bottom surface causes a change in the
propagating light. The detection arrangement comprises a light detector
arranged to detect the change in the propagating light. A compact and
robust incoupling site is defined by a sheet-like micro-structured
surface portion which is fixedly arranged on one of the top and bottom
surfaces and configured to transmit light having an angle of incidence
that enables light propagation by TIR inside the panel.

Claims:

1. A touch-sensitive apparatus, comprising: a light transmissive panel
that defines parallel top and bottom surfaces; an illumination
arrangement configured to couple light into the panel via an incoupling
site such that the light propagates by total internal reflection in at
least one of the top and bottom surfaces and such that an object touching
said at least one of the top and bottom surfaces causes a change in the
propagating light; a detection arrangement comprising a light detector
arranged to detect said change in the propagating light; wherein the
incoupling site is defined by a sheet-like micro-structured surface
portion which is fixedly arranged on one of the top and bottom surfaces
and configured to transmit light having an angle of incidence that
enables light propagation by total internal reflection inside the panel.

2. The apparatus of claim 1, wherein the micro-structured surface portion
is included in a thin film attached to the panel.

3. The apparatus of claim 2, wherein the thin film comprises a sheet
substrate of plastic material and facets formed by structuring and curing
a resin on the sheet substrate.

4. The apparatus of claim 3, wherein the thin film comprises an adhesive
backing opposite to the facets.

5. The apparatus of claim 2, wherein the incoupling site is elongate and
defined by a sequence of separate thin films arranged side by side in a
longitudinal direction of the incoupling site.

6. The apparatus of claim 2, wherein the thin film comprises at least one
line of weakness that extends in a transverse direction of the incoupling
site.

7. The apparatus of claim 1, wherein the micro-structured surface portion
is configured to project from said one of the top and bottom surfaces by
less than 1 mm.

8. The apparatus of claim 1, wherein the micro-structured surface portion
comprises a plurality of parallel elongate facets that collectively mimic
an essentially continuous input surface for receiving the light to be
coupled into the panel.

9. The apparatus of claim 8, wherein all of the facets have essentially
the same angle to a normal of the top and bottom surfaces.

10. The apparatus of claim 8, wherein each facet is bounded by elongate
release surfaces, which extend parallel to the facet and are essentially
perpendicularly inclined to the facet.

11. The apparatus of claim 8, wherein the elongate facets extend in a
longitudinal direction and are distributed in a transverse direction of
the micro-structured surface portion, and wherein the illumination
arrangement is configured to direct light onto the micro-structured
surface portion such that the micro-structured surface portion receives
light with a main direction that is essentially perpendicular to the
facets at least in the transverse direction.

12. The apparatus of claim 8, wherein at least a subset of the facets
have an individual inclination such that the micro-structured surface
portion has an optical power that affects the transmitted light.

13. The apparatus of claim 8, wherein at least a subset of the facets is
provided with a diffusing surface structure.

15. The apparatus of claim 8, wherein at least a subset of the facets is
designed to separate incoming light into a given number of distinct beams
with different main directions in the plane of the panel.

16. The apparatus of claim 15, wherein the facets are provided with a
diffractive grating structure for separating the incoming light into the
given number of distinct beams.

17. The apparatus of claim 1, wherein the illumination arrangement is
configured to direct light onto the micro-structured surface portion such
that the micro-structured surface portion receives collimated light in a
transverse direction of the incoupling site.

18. The apparatus of claim 1, wherein the illumination arrangement is
configured to direct light onto the micro-structured surface portion such
that the micro-structured surface portion receives divergent light in a
transverse direction of the incoupling site.

19. The apparatus of claim 1, wherein the micro-structured surface
portion is integrated in the panel.

20. An optical coupling element for coupling light into a light
transmissive panel in a touch-sensitive apparatus, wherein the light
transmissive panel comprises parallel top and bottom surfaces and is
arranged to propagate light by total internal reflection from an
incoupling site to an outcoupling site, wherein the optical coupling
element is formed as a film of light transmissive plastic material, which
film comprises a micro-structured surface portion and an attachment
surface opposite to the micro-structured surface portion, the film being
adapted for attachment to one of the top and bottom surfaces so as to
form at least part of the incoupling, by the micro-structured surface
portion being arranged to transmit light that has an angle of incidence
enabling light propagation by total internal reflection inside the panel.

21. An optical touch panel for a touch-sensitive apparatus, wherein the
optical touch panel comprises parallel top and bottom surfaces and is
arranged to propagate light by total internal reflection from an
incoupling site to an outcoupling site, wherein the optical touch panel
further comprises a micro-structured surface portion which is formed on
one of the top and bottom surfaces to define the incoupling site and
which is configured to transmit light having an angle of incidence that
enables light propagation by total internal reflection inside the panel.

[0002] The present invention relates to touch-sensitive systems that
operate by light transmission through light transmissive panels, and in
particular to optical solutions for coupling light into such panels.

BACKGROUND ART

[0003] The prior art comprises different types of touch-sensitive systems
that operate by transmitting light inside a solid light transmissive
panel, which is defined by two parallel boundary surfaces connected by a
peripheral edge surface. Specifically, light is injected into the panel
so as to propagate by total internal reflection (TIR) between the
boundary surfaces. An object that touches one of the boundary surfaces
("the touch surface") causes a change in the propagating light that is
detected by one or more light sensors. In one implementation, e.g. as
disclosed in WO2008/017077, US2009/267919 and WO2010/056177, light
sensors are arranged behind the panel to detect light which scatters off
the touching object and escapes the panel via the boundary surface
opposite to the touch surface. In another implementation, e.g. as
disclosed in U.S. Pat. No. 7,435,940, light sensors are arranged at the
periphery of the panel to detect light which scatters off the touching
object and is confined within the panel by total internal reflection. In
yet another implementation, e.g. as disclosed in WO2010/006882 and
WO2010/134865, light sensors are arranged at the periphery of the panel
to sense the attenuation of the light transmitted through the panel.

[0004] In order to achieve a uniform illumination of the touch surface
from within the panel, the incoupling site is often elongate and extends
along a significant portion of the panel. Most prior art documents
propose injecting the light through an elongate portion of the peripheral
edge surface, without any dedicated coupling elements. Such an approach
is possible since the light can be injected at a relatively steep angle
to the edge surface, resulting in comparatively small reflection losses
at the edge surface. Also, such an incoupling site does not add
significantly to the thickness of the touch system. However, incoupling
via the edge surface requires the edge surface to be highly planar and
free of defects. This may be difficult and/or costly to achieve,
especially if the panel is thin and/or manufactured of a comparatively
brittle material such as glass. Incoupling via the edge surface may also
add to the footprint of the touch system. Furthermore, it may be
difficult to optically access the edge surface if the panel is attached
to a mounting structure, such as a frame or bracket, and it is also
likely that the mounting structure causes strain in the edge surface.
Such strain may affect the optical quality of the edge surface and result
in reduced incoupling performance.

[0005] Above-mentioned WO2010/006882 and WO2010/134865 propose incoupling
of light via elongate wedges that are attached and optically coupled
(glued) to the top or bottom surfaces. Such an approach may mitigate any
strict requirements for the surface properties of the edge surface and/or
facilitate mounting of the panel. However, in order to achieve a uniform
illumination of the touch surface by propagating light that is collimated
in the depth direction of the panel, the incoupling site needs to admit a
beam of light with such an extent in the depth direction of the panel
that the footprint of the beam on the touch surface essentially overlaps
between successive reflections in the touch surface. This, in turn, means
that the wedge needs to have a light-receiving surface of corresponding
dimensions, resulting in a wedge that may need to project at least 15-20
mm from the top or bottom surface. Such a wedge may add significant
thickness and weight of the system. To reduce weight and cost, the wedge
may be made of plastic material. On the other hand, the panel is often
made of glass, e.g. to attain required bulk material properties (e.g.
index of refraction, transmission, homogeneity, isotropy, durability,
stability, etc) and surface evenness of the top and bottom surfaces. The
present applicant has found that the difference in thermal expansion
between the plastic material and the glass may cause the wedge to come
loose from the panel as a result of temperature variations during
operation of the touch system. Even a small or local detachment of the
wedge may cause a significant decrease in the performance of the system.

[0006] The present applicant has tried to overcome this problem by
attaching several shorter wedges side-by-side so as to form the elongate
incoupling site. However, if the touch system requires more than one
sheet of light to be injected via the incoupling site, such that the
light transmitted via the incoupling site has more than one main
direction in the plane of the panel (i.e. as seen in a plan view of the
touch surface), the joints between the wedges may interfere with (e.g.
reflect) the incoming light and cause a significantly reduced performance
of the system. For example, WO2010/006882 and WO2010/134865 disclose
techniques for enabling multi-touch sensitivity by injecting plural
sheets with different main directions via the incoupling site.

[0007] The prior art also comprises US2004/0252091, which discloses an
optical touch system in which diverging light beams are coupled into a
light transmissive panel for propagation by TIR via large wedges in the
form of revolved prisms that are arranged on the top or bottom surface of
the panel.

[0008] Outside the field of optical coupling elements for touch systems,
it is known to provide flat panel displays with a so-called brightness
enhancement film (BEF), which is a transparent optical film designed to
increase the display brightness through improved light management, see
e.g. US2005/248848 and US2010/259939. Specifically, the BEF is a
micro-structured sheet with a plurality of prismatic and/or lenticular
elements and may be adhered to a light-transmissive substrate in the
display. The micro-structures are designed to increase the spatially
averaged luminance of the display in a certain range of angles around a
normal (perpendicular) viewing direction.

[0009] U.S. Pat. No. 6,972,753 discloses an optical touch panel in which a
BEF-type element ("a prism lens sheet") is attached to a light source
arranged alongside a light guide panel, so as to enhance the directivity
of the emitted light before the light is directed onto the edge surface
of the light guide panel. The prism lens sheet is not a coupling element,
but rather an upstream collimator that ensures that all light is injected
in a single, well-defined main direction in the plane of the panel.

[0010] U.S. Pat. No. 6,803,900 discloses a lighting system for an LCD
display. The lighting system comprises a side-illuminated flat light
guide with micro-optical structures on its top surface that give a
preferential outcoupling of light. A light pipe is disposed parallel to
the light guide to couple light into the light guide through its
peripheral edge surface. The light pipe is provided with micro-optical
surface structures which cause light, which is guided inside the light
pipe from one end towards the other end, to be re-directed towards the
peripheral edge surface of the light guide.

[0011] WO2007/112742 discloses an optical touch pad in which a beam
expander is arranged intermediate an emitter of collimated light and the
edge surface of a light transmissive panel. A beam splitter in the form
of a plurality of prisms is formed on the edge surface to receive the
expanded beam and divide it into two expanded collimated beams with
controlled angles of incidence inside the panel. The beam splitter is
only useful on the edge surface since it is designed to produce two beams
with different directions in the depth direction of the panel.

SUMMARY

[0012] It is an object of the invention to at least partly overcome one or
more of the above-identified limitations of the prior art. One objective
is thus to provide an efficient and robust coupling of light into a light
transmissive panel of an optical touch system, which is less dependent on
the quality of the edge surface and which allows the light to propagate
by total internal reflection inside the panel.

[0013] Yet another objective is to provide an incoupling site that adds
little weight and size to the touch system.

[0014] A further objective is to allow the light transmitted via the
incoupling site to have more than one main direction in the plane of the
panel.

[0015] These and other objects, which may appear from the description
below, are at least partly achieved by means of a touch-sensitive
apparatus, an optical coupling element and an optical touch panel
according to the independent claims, embodiments thereof being defined by
the dependent claims.

[0016] A first aspect of the invention is a touch-sensitive apparatus
which comprises: a light transmissive panel that defines top and bottom
surfaces; an illumination arrangement configured to couple light into the
panel via an incoupling site such that the light propagates by total
internal reflection in at least one of the top and bottom surfaces and
such that an object touching said at least one of the top and bottom
surfaces causes a change in the propagating light; and a detection
arrangement comprising a light detector arranged to detect said change in
the propagating light; wherein the incoupling site is defined by a
sheet-like micro-structured surface portion which is fixedly arranged on
one of the top and bottom surfaces and configured to transmit light
having an angle of incidence that enables light propagation by total
internal reflection inside the panel.

[0017] By implementing the incoupling site as a sheet-like microstructured
surface portion arranged on the top or bottom surface of the light
transmissive panel, light can be coupled into the panel irrespective of
the quality of the edge surface of the panel and without the need for a
coupling element that adds significant weight and/or size to the
touch-sensitive apparatus. As used herein, a "microstructured surface"
contains surface structures having at least one dimension in the range of
0.1-1000 μm. Microstructures and microstructured surfaces are
well-known per se in the field of optical technology. In one example, the
microstructured surface portion comprises a plurality of microreplicated
prismatic elements that collectively define a light-receiving surface
with a suitable inclination to the light that is to be transmitted by the
surface portion (and thereby coupled into the panel). Specifically, the
light is transmitted with an angle of incidence, given with respect to
the normal of the top and bottom surfaces, that exceeds the critical
angle for total reflection in the panel. Each prismatic element may
include a light-receiving facet which is designed such that the facets of
the surface portion collectively define a coherent or continuous light
receiving surface to the light that is to be coupled into the panel. A
"facet" is a continuous surface unit of a prismatic element, as is
well-known to the person skilled in the art. Each facet may extend in a
longitudinal direction of the microstructured surface portion, and the
facets may be formed side by side in a transverse direction of the
microstructured surface portion. Thereby, the microstructured surface
portion may be designed to mimic a light-receiving front surface similar
to the planar light-receiving surface of a conventional wedge, but
without the need for the microstructured surface portion to project
significantly from the top or bottom surface. In fact, the
microstructured surface portion may have an essentially flat
configuration on the top or bottom surface, and may be designed to
project 1 mm or less from the top or bottom surface.

[0018] The use of a sheet-like surface portion enables simple integration
of the surface portion into the top or bottom surface of the panel, by
the microstructure being directly formed in the top or bottom surface, so
as to define a robust and durable incoupling site.

[0019] Another simple way of defining a compact and robust incoupling site
is to manufacture and attach a dedicated sheet-like incoupling element,
which defines the microstructured surface portion, to the top or bottom
surface of the light transmissive panel. Such a sheet-like incoupling
element may be so thin and flexible that it is able to absorb any shear
forces that occur in the interface between the incoupling element and the
light transmissive panel, e.g. caused by the above-mentioned difference
in thermal expansion. Thereby, the sheet-like incoupling element may be
firmly and robustly attached to the panel.

[0020] In one embodiment, the thickness of the sheet-like incoupling
element is less than about 1/10 of the thickness of the light
transmissive panel, and more preferably less than about 1/20, 1/30, 1/40
or 1/50 of the thickness of the light transmissive panel. However, the
skilled person realizes that the thickness of the sheet-like incoupling
element may be optimized in view of the material and structure of the
sheet-like element, the expected amount of shear forces, the required
durability of the incoupling site, manufacturability issues, etc.

[0021] As a further advantage over a conventional wedge, the inventive
implementation of the incoupling site enables additional optical
functionality to be embedded in the microstructured surface portion, at
low cost and little added complexity. For example, the microstructured
surface portion may embed at least one of a refractive function, a
diffractive function and a diffusive function into its light receiving
surface, e.g. by proper design of the above-mentioned facets, and it may
even be possible to tailor this additional optical functionality in
different parts of the light receiving surface. The provision of such
additional optical functionality may further improve the performance of
the touch-sensitive apparatus, at the cost of minimal structural changes.

[0022] It should be understood that the touch-sensitive apparatus may
employ any detection strategy for determining touch data for touching
objects. Such detection strategies include, as discussed in the
Background section, detecting the energy (or equivalently, power or
intensity) of light scattered by the touching objects or detecting the
remaining energy (or equivalently, power or intensity) of the propagating
light downstream of the touching object.

[0023] A second aspect of the invention is an optical coupling element for
coupling light into a light transmissive panel in a touch-sensitive
apparatus, wherein the light transmissive panel comprises top and bottom
surfaces and is arranged to propagate light by total internal reflection
from an incoupling site to an outcoupling site, wherein the optical
coupling element is formed as a film of light transmissive plastic
material, which film comprises a micro-structured surface portion and an
attachment surface opposite to the micro-structured surface, the film
being adapted for attachment to one of the top and bottom surfaces so as
to form at least part of the incoupling site, by the micro-structured
surface portion being arranged to transmit light that has an angle of
incidence enabling light propagation by total internal reflection inside
the panel. Such an optical coupling element is a light-weight component
that may be conveniently attached to the top and/or bottom surface of a
light transmissive panel so as to define a space-efficient and robust
incoupling site that is able to efficiently couple light into the panel
for propagation by TIR.

[0024] A third aspect of the invention is an optical touch panel for a
touch-sensitive apparatus, wherein the optical touch panel comprises top
and bottom surfaces and is arranged to propagate light by total internal
reflection from an incoupling site to an outcoupling site, wherein the
optical touch panel further comprises a micro-structured surface portion
which is formed on one of the top and bottom surfaces to define the
incoupling site and which is configured to transmit light having an angle
of incidence that enables light propagation by total internal reflection
inside the panel. Such an optical touch panel may be provided as a
unitary component for installation in the touch-sensitive apparatus. The
incoupling site may be defined to efficiently couple light into the panel
for propagation by TIR by means of the microstructured surface portion
which may be implemented as a space-efficient and robust feature on the
top and/or bottom surface.

[0025] The second and third aspect share the advantages and technical
effects of the first aspect. Any one of the embodiments of the first
aspect, as outlined above and as further exemplified in the following
detailed description, as defined in the dependent claims and as
illustrated on the drawings, may be combined with the second and third
aspects.

[0026] Still other objectives, features, aspects and advantages of the
present invention will appear from the following detailed description,
from the attached claims as well as from the drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0027] Embodiments of the invention will now be described in more detail
with reference to the accompanying schematic drawings.

[0028]FIG. 1 is a top plan view of a touch-sensitive apparatus according
to an embodiment of the invention.

[0029]FIG. 2 is an enlarged side view illustrating light paths at an
incoupling site of the apparatus in FIG. 1.

[0030]FIG. 3 illustrates a comparison between an embodiment of the
invention and a conventional coupling element.

[0031] FIG. 4 is a top plan view of a touch panel with elongate incoupling
and outcoupling sites.

[0032]FIG. 5 is a top plan view of a coupling tape according to one
embodiment.

[0033]FIG. 6 is a top plan view of a coupling tape according to another
embodiment.

[0034]FIG. 7 is a side view of a production plant for manufacturing a
micro-structured sheet for use in producing the coupling tapes in FIGS. 5
and 6.

[0035]FIG. 8 is a top plan view of a sheet roll produced in the plant of
FIG. 7.

[0036] FIG. 9A is a perspective view of a coupling element according to
one embodiment, and FIG. 9B is a top plan view of the coupling element in
FIG. 9A as installed in a touch-sensitive apparatus.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0037] For the sole purpose of explaining principles of the present
invention, the following disclosure will be given with respect to a
specific type of touch-sensitive apparatus or touch system in which beams
are injected into and swept across a light transmissive panel, and
touching objects are detected based on the amount of light that is
received at an opposite end of the panel. This type of touch system is
also denoted "scanning FTIR system" herein.

[0038] Throughout the description, the same reference numerals are used to
identify corresponding elements.

[0039]FIG. 1 is a top plan view of a scanning FTIR system. The system
includes a light transmissive panel 1, two light-emitting input scanners
2A, 2B and two light sensors 3A, 3B. The panel may be planar or curved
and defines two opposite and generally parallel surfaces 4, 5 (see FIG.
2), which are connected by a peripheral edge surface 1' (FIG. 2). A
radiation propagation channel is provided between the top and bottom
surfaces 4, 5 of the panel 1, wherein at least one of the surfaces 4, 5
("the touch surface") allows the propagating light to interact with a
touching object O1. Typically, the light from the input scanners 2A, 2B
is injected to propagate by total internal reflection (TIR) in the
radiation propagation channel, and the sensors 3A, 3B are arranged
downstream the propagation direction to receive the light and generate a
respective measurement signal which is indicative of the energy of
received light. The phenomenon of TIR is well known in the art and will
not be explained further.

[0040] Generally, the panel may be made of any material that transmits a
sufficient amount of radiation in the relevant wavelength range to permit
a sensible measurement of transmitted energy. Such material includes
glass, poly(methyl methacrylate) (PMMA) and polycarbonates (PC).

[0041] It should be noted that the propagating light needs only be
reflected by TIR in the touch surface, and that the opposite surface may
be provided with a reflective coating. However, if both surfaces 4, 5 are
to be used as touch surfaces, or if the touch system needs to be
transparent for viewing through the surfaces 4, 5, the touch system may
be designed to propagate the light by TIR in both surfaces 4, 5. If the
touch system needs to be transparent for viewing through the surfaces 4,
5, the bottom surface 5 may instead be provided with a coating that is
transparent for viewing at right angles, but which is reflective to light
propagating inside the panel. For example, such a coating may be designed
to transmit visible light while reflecting infrared light, at least for a
range of angles of incidence.

[0042] In FIG. 1, each of the input scanners 2A, 2B is arranged to
generate and sweep a beam of light along an elongate fixed re-directing
element 4A, 4B that is designed and arranged to output a beam with a
desired direction. Downstream of the re-directing element 4A, 4B, the
beams are swept along elongate incoupling sites 5A, 5B on the bottom
surface 5 (FIG. 2) of the panel 1. The combination of input scanners 2A,
2B, re-directing elements 4A, 4B and incoupling sites 5A, 5B forms an
illumination arrangement which is configured to sweep two beams B1, B2
inside the panel in two different directions R1, R2. Each beam B1, B2 is
suitably collimated at least along its main direction in the plane of the
panel (x,y plane), and may or may not be collimated also in the depth
direction (i.e. transverse to the plane of the panel). The sweeping of
each beam B1, B2 serves to form a respective curtain of light on the
touch surface from within the panel. Each such light curtain illuminates
the touch surface, or part thereof. The light may be generated by any
type of light source capable of emitting light in the desired wavelength
range, for example a diode laser, a VCSEL (vertical-cavity
surface-emitting laser), or an LED (light-emitting diode), an
incandescent lamp, a halogen lamp, etc.

[0043] The received energy along an elongate outcoupling site 6A, 6B on
the bottom surface 5 of the panel 1 is measured by the sensors 3A, 3B
which are arranged to receive the beams B1, B2 as they are swept inside
the panel 1. The sensors 3A, 3B may include or be part of any type of
device capable of converting light into an electrical measurement signal,
e.g. a photo-detector, a CCD or CMOS detector, etc.

[0044] In the specific example of FIG. 1, a fixed elongate re-directing
device 7A, 7B is arranged to receive and re-direct the outcoupled beams
B1, B2 onto a common detection point D1, D2 while the beams B1, B2 are
swept across the panel 1. The combination of outcoupling sites 6A, 6B,
re-directing elements 7A, 7B and sensors 3A, 3B forms a detection
arrangement. The re-directing devices 4A, 4B, 7A, 7B may operate by
reflection or transmission of light and may be made up of diffractive
optical elements (DOE), micro-optical elements, mirrors, refractive
lenses, and any combination thereof. In one embodiment, the re-directing
devices 4A, 4B, 7A, 7B are configured as Fresnel cylindrical lenses.

[0045] When the object O1 is brought sufficiently close to the touch
surface (in this example, the top surface 4), the total internal
reflection is frustrated and the energy of the transmitted light is
decreased ("attenuated"). In FIG. 1, a controller 8 is arranged to
control the operation of the input scanners 2A, 2B, and a data processor
9 is configured to obtain and process time-resolved measurement signals
from the sensors 3A, 3B to determine touch data for the touching object
O1 within the sensing area. The touch data may, e.g., represent one or
more of the location, shape and size of the object O1. The touch data may
be determined by triangulation or more advanced processing techniques,
including tomographic reconstruction methods such as FBP (Filtered Back
Projection), ART (Algebraic Reconstruction Technique), SART (Simultaneous
Algebraic Reconstruction Technique), etc. Further examples of advanced
data processing techniques designed for use in touch determination are
found in WO 2010/006883, WO2009/077962, WO2011/049511, and WO2011/139213,
all of which are incorporated herein by reference.

[0046] In the example of FIG. 1, the respective beam B1, B2 has an
essentially invariant main direction in the plane of the panel during the
sweep. The "main direction" of the beam is the propagation direction of
the beam as projected onto the touch surface 4. The main direction is
also denoted "scan angle" herein. In the illustrated example, the main
directions of the beams B1, B2 are essentially parallel to a respective
edge of the panel 1. However, as explained in WO2010/006882, the number
of beams, their mutual angle, and their angle with respect to the edges
of the panel, may be configured otherwise in order to achieve various
technical effects, e.g. multi-touch functionality.

[0047]FIG. 2 is a side view of the incoupling site 5B in FIG. 1 as it
receives an incoming beam which is collimated in the transverse direction
of the incoupling site 5B. The incoupling site 5B is defined by an
essentially flat coupling element 20 which is attached to the bottom
surface 5 by means of an adhesive layer 21 that is transparent to the
light generated by the input scanner 2B (FIG. 1). The coupling element 20
comprises a surface structure 22 which is formed of a light transmissive
material and comprises, in the transverse direction of the coupling
element 20, an alternating sequence of light-receiving facets 23 and
release surfaces 24 (also denoted relief surfaces). In the example of
FIG. 2, the surface structure 22 is joined to a transparent substrate 25.

[0048] It should be understood that the facets 23 and release surfaces 24
extend parallel to each other in the longitudinal direction of the
coupling element 20, as indicated by thin lines in the plan view of FIG.
4. In the example of FIG. 2, the incoming light is directed essentially
normal to the facets 23 in the transverse direction of the coupling
element 20. This may serve to reduce or minimize the impact of tolerances
in the inclination of the facets 23 on the direction of the transmitted
light. Furthermore, reflection losses in the facets 23 are minimized,
e.g. if the incoming light is unpolarized. If the incoming light is a
single collimated beam which is swept along the coupling element 20 at an
essentially invariant scan angle (cf. FIG. 1), the beam is preferably
directed normal to the facets 23 also in the longitudinal direction of
the coupling element 20. If plural collimated beams with different scan
angles are coupled into the panel 1 via the coupling element 20, only one
of the beams may be normal to the facets 23 in the longitudinal
direction, whereas all beams preferably are normal to the facets 23 in
the transverse direction. This will ensure that the beams are given the
same bounce angle (see below) inside the panel, and may enable a
simplified configuration of the illumination arrangement. Likewise, if
the incoming light is collimated in the transverse direction and
divergent in the longitudinal direction, the incoming light is preferably
directed normal to the facets 23 in the transverse direction.

[0049] The release surfaces 24, which may also be denoted "draft facets",
are designed so as not to be illuminated by the incoming light and
thereby not affect the optical performance of the coupling element. In
the illustrated example, the release surfaces 24 are essentially
perpendicular to the facets 23, although other release surface designs
are conceivable.

[0050] As shown in FIG. 2, the coupling element 20 transmits the incoming
light such that it hits the top surface 4 at an angle of incidence
θ to the normal N of the surface 4. The angle of incidence θ
is also denoted "bounce angle" herein. The light is directed onto the
coupling element 20 such that the bounce angle θ exceeds the
so-called critical angle θc, which is the minimum angle to
sustain total internal reflection. For a boundary between panel material
and air, the critical angle θc is approximately 42°,
assuming an index of refraction of 1.5 for the panel material. It may be
preferable for the bounce angle to exceed about 60°, since the
present Applicant has found that water deposits on either of the surfaces
4, 5 may result in a significant frustration of the total internal
reflection for bounce angles below about 60°, whereas the
frustration may be less for larger bounce angles. Furthermore, it has
been found that the frustration caused by human fingers touching the
panel material may decrease for bounce angles above about 70°.
Thus, by designing the illumination arrangement to generate bounce angles
in the approximate range of 60°-70°, it is possible to
minimize the unwanted influence of deposits on the touch surface while
maximizing the desired influence of touching objects. Presently, the most
preferred range of bounce angles is in the range of
64°-68°.

[0051] Although not shown in FIG. 2, it is to be understood that the
transmitted light may be refracted in any one of the boundaries between
the structure 22 and the substrate 25, between the substrate 25 and the
adhesive layer 21, and between the adhesive layer 21 and the panel 1, due
to differences in refractive index across these boundaries. The skilled
person realizes that the angle of the facets 23 and/or the inclination of
the incoming light may be adapted in view of such refraction, to achieve
a desired bounce angle θ inside the panel 1.

[0052] The illumination arrangement is designed to illuminate the touch
surface 4 from within the panel 1. Preferably, the illumination
arrangement should be designed to avoid that an incoming beam bounces
inside the panel 1 such that it illuminates the touch surface 4 in
spatially separated regions. This may be avoided by requiring the
incoming beam to at least have a minimum extent in the transverse
direction of the coupling element (x direction in FIG. 2), such that
there is no gap between the footprint F2 of the beam B2 at successive
reflections in the touch surface 4. The minimum extent is a function of
the bounce angle and the thickness of the panel 1. To attain a uniform
illumination of the touch surface 4, the illumination arrangement may be
designed to generate a beam of light that is collimated in the depth
direction (i.e. as seen in the x,z plane of FIG. 2) and propagates by TIR
to produce footprints F2 that are placed edge to edge along the touch
surface 4. In such an example, the minimum extent is given by F2=2D tan
θ, where θ is the bounce angle, and D is the thickness of the
panel 1 (in the z direction). When the footprints F2 are placed edge to
edge, the energy distribution of the footprint F2 (in the x direction)
may be a "top hat" function, i.e. an essentially even distribution of
energy bounded by steep gradients.

[0053] To accommodate for tolerances in the illumination arrangement
and/or the panel 1, the illumination arrangement may instead be nominally
designed to generate an overlap between successive footprints F2. In such
a variant, the individual footprint F2 may have a "smooth" energy
distribution, namely such an energy distribution that the combination of
overlapping footprints F2 produces a relatively uniform illumination of
the touch surface. Such a smooth footprint F2 has end portions with
gradually decreasing energy, and may e.g. have an energy distribution (in
the x-direction) that resembles a Gaussian function. To this end, the
incoming light may be given a desirable beam profile by the light source
or by a dedicated optical device in the illumination arrangement, and/or
the energy distribution of the incoming light may be modified by
diffusing structures embedded in the coupling element 20, as discussed
further below.

[0054] Furthermore, to reduce the need to accurately center the incoming
light on the coupling element 2, or vice versa, the illumination
arrangement may be designed to generate the incoming light that has a
larger extent in the transverse direction than the coupling element 20.

[0055] It may be desirable to use divergent light in the illumination
arrangement, i.e. upstream of the panel 1, for example to reduce the
required size of optical components (mirrors, lenses etc) used for
directing the light from the input scanner 2A, 2B to the incoupling site
5A, 5B. In one embodiment, the coupling element 20 is designed with an
optical power such that divergent incoming light is essentially
collimated in the depth direction when it is transmitted into the panel
1. The optical power may be obtained by designing each facet 23 with a
dedicated inclination such that the resulting light refraction by all
facets 23 collectively collimates the incoming divergent light. In an
alternative embodiment, incoming divergent light is coupled into the
panel 1 so as to diverge in the depth direction as it propagates by total
internal reflection. In such an embodiment, to minimize reflective losses
in the coupling elements 20 and/or refraction of the transmitted light,
the facets 23 may be individually angled so as to be essentially
perpendicular to the incoming light.

[0056] Generally, the coupling element 20 may be designed with an optical
power that generates a desired change of a beam property of the incoming
light, e.g. such that the transmitted light has a certain divergence or
convergence, or is collimated, in one or more directions (e.g. in the x,z
plane or in the x,y plane in FIGS. 1-2)

[0057] It should be understood that the size of the coupling element 20
and its surface structure 22 is exaggerated in FIG. 2. In practical
implementations, the surface structure is typically a micro-structure
that projects less than 1 mm from the panel surface 5. For example, the
substrate may have a thickness of about 50-300 μm, and the surface
structure may project about 20-100 μm from the substrate. Also, the
number of facets 23 in the transverse direction may be larger than shown
in FIG. 2. The spacing of facets 23 may be given as a pitch, which is the
extent of one facet 23 and one release surface 24 in the transverse
direction of the coupling element 20. The pitch is a function of the
thickness of the structure 22 and the angle of the facets 23. Typically,
the pitch is in the range of about 50-500 μm.

[0058]FIG. 3 is a side view of a coupling element 20 similar to the one
in FIG. 2 and is intended to show that the coupling element 20 is
designed to correspond to a wedge 30 of solid transparent material, as
used in the prior art. Such a wedge 30 is indicated by dotted lines in
FIG. 3. By proper arrangement of the facets and the release surfaces, the
facets collectively define an input surface for the incoming light, which
functionally corresponds to the front light-receiving surface 31 of the
wedge 30. Thus, the micro-structured, sheet-like coupling element 20 is
capable of replacing the conventional wedge 30. The input surface of the
coupling element 20 is thus made up of the facets 23, which are designed
such that the illuminated portions of the facets 23 act as a continuous
or coherent surface of proper inclination for the incoming light.
Although the illustrated examples represent a coupling element 20 with
straight elongate facets 23, it is to be understood that the facets may
be curved, e.g. to couple light from a point-like light source into the
panel. For example, such curved facets may be designed to mimic the
curved input surface of the incoupling wedge as disclosed in
US2004/0252091.

[0059] FIG. 4 is a top plan view of an optical panel component for the
touch system in FIG. 1. The optical panel component is formed by the
light transmissive panel 1, the incoupling elements 20 that are provided
on the bottom surface 5 to define the incoupling sites 5A, 5B, and the
outcoupling elements 40 that are provided on the bottom surface 5 to
define the outcoupling sites 6A, 6B. It is to be understood that the
incoupling and/or outcoupling elements 20, 40 may alternatively or
additionally be provided on the top surface 4. It is also conceivable
that the outcoupling sites 6A, 6B are formed on the edge surface (cf. 1'
in FIG. 2). As noted above, the facets 23 are mutually parallel and
extend in the longitudinal direction of the incoupling element 20, such
that the incoupling element 20 has the same optical properties along its
extent. Each incoupling element 20 may be formed as a separate sheet-like
component which is optically coupled and fixedly attached to the surface
of the panel 1. In one embodiment, the incoupling element 20 is in the
form of a micro-structured tape with an adhesive backing ("a coupling
tape"). Alternatively, the incoupling elements 20 may be implemented as
structures integrated in the surface of the panel 1. The specific design
of the illustrated touch system (FIG. 1), namely the similarity between
the illumination arrangement and the detection arrangement, may allow the
outcoupling elements 40 to be similar, or even identical, to the
incoupling elements 20. FIG. 4 also indicates the sensing area 45 of the
touch system, which is the area where touching objects may be detected.
Typically, the sensing area 45 is defined as the surface area of the
touch surface that is illuminated by at least two overlapping sheets of
light with different scan angles.

[0060] By designing the incoupling element 20 with a micro-structured
surface, the weight and size of the incoupling element 20 may be reduced
to a minimum. If the micro-structured surface is integrated into the top
or bottom surface 4, 5 of the panel 1, the risk for detachment of the
incoupling element 20 from the panel 1 is eliminated. If the
micro-structured surface is implemented as a thin, sheet-like component
that is attached to the panel surface, the reduced thickness of the
component allows the component to deform and thereby absorb shear forces
that are formed in the joint between the component and the panel surface,
e.g. because of differences in thermal expansion. Thereby, the risk for a
local detachment of the incoupling element is significantly reduced
compared to a wedge-shaped incoupling element as indicated in FIG. 3.

[0061] The ability of absorbing shear forces may be further improved by
providing the incoupling sites 5A, 5B with force absorbing portions that
extend in the transverse direction. FIG. 5 illustrates an embodiment in
which a sequence of coupling elements 20 are arranged one after the other
along the incoupling site 5A. The transverse ends are arranged in
abutting relationship, forming a border 50 between the elements 20, such
that shear forces may act to move the coupling elements 20 slightly apart
in the longitudinal direction of the incoupling site 5A, thereby reducing
the shear forces in the adhesive layer (cf. 21 in FIG. 2). FIG. 6
illustrates an embodiment in which a unitary coupling element 20 is
provided with transverse lines of weakness 60, e.g. cut-outs or thinned
portions, which serve to absorb shear forces. It is to be understood that
the abutting ends (FIG. 5) and the lines of weakness 60 (FIG. 6) may be
designed with any angle with respect to the longitudinal direction of the
incoupling site 5A. As used herein, a "line of weakness" denotes an
elongated portion of weakened material, which may be created by locally
thinning the material, by providing cuts, scores or perforations in the
material, or by otherwise processing the material to locally decrease its
strength.

[0062] As mentioned above, beams may be swept inside the panel at any scan
angle, e.g. at a scan angle that is non-perpendicular to the extent of
the incoupling site 5A, 5B. As noted in the Background section, such
non-perpendicular scan angles require the light to pass the incoupling
site 5A, 5B at an angle to its transverse direction and may thus be
reflected or otherwise disturbed by the borders 50 in FIG. 5 and the
lines of weakness 60 in FIG. 6, respectively. However, since the coupling
element 20 is implemented as a thin sheet-like component, these borders
50 and lines of weakness 60 have a minute surface area and thus have a
minimal influence on the transmitted light.

[0063] The surface structure 22 of the coupling element 20 may be
manufactured using any known technique for replicating micro-structures,
including compression molding, injection molding, hot embossing, and
wafer level optics. In such processes, the structures are replicated onto
a substrate which, as indicated above, may be the panel surface or a
separate sheet material.

[0064]FIG. 7 is a schematic illustration of another technique for
mass-production of the above-mentioned coupling tape. A web 70 of thin,
flexible sheet material is continuously or intermittently fed through a
manufacturing plant for processing in a sequence of stations 71-76. In
station 71, a photo-curable resin composition is applied to the surface
of the web 70. The resin composition is selected to be curable to a state
that is flexible with respect to the final thickness of the resulting
surface structure (cf. thickness of structure 22 in FIG. 2). Examples of
resin compositions include, but are not limited to, acrylate, epoxy and
urethane based materials. In station 72, the web 70 is passed through a
pair of rotating rollers 77, 78, which replicate a microstructure in the
resin. The roller 77 has a master negative microstructure molding pattern
77' on its outer surface, and the roller 78 has a smooth periphery. The
molding pattern 77' is a negative of the facets 23 and release surfaces
24 (FIG. 2) and may be produced by diamond cutting either an array of
parallel discrete circumferential grooves on the roller 77 or a single
continuous groove that spirals the circumference of the roller 77. In
station 73, the replicated micro-structure in the resin is cured by
exposing the resin to radiation, e.g. ultraviolet radiation. In station
74, an adhesive is applied to the lower side of the web 70 to form an
adhesive backing. In station 75, a web 79 of thin plastic or paper
material is joined with the web 70 to form a protective sheet for the
adhesive backing. In station 76, the micro-structured sheet is winded
into a roll 80.

[0065]FIG. 8 is a plan view of the roll 80 with an end portion rolled out
to display the surface structure 22 of the micro-structured sheet, where
the parallel lines represent apices between facets and release surfaces
(with greatly exaggerated spacing). Dashed lines 81 indicate a cut for
extracting a coupling element 20 from the micro-structured sheet. It is
to be understood that station 76 in FIG. 7 may be replaced by a station
for cutting coupling elements from the micro-structured sheet.

[0066] Returning to FIG. 2, it has already been noted that the facets 23
may be provided with an optical power, by designing the facets to
individually refract the incoming light. The facets 23 may alternatively,
or additionally, be provided (embedded) with other functional structures.
In one example, the facets 23 (or a subset thereof) incorporate light
diffusing structures, i.e. structures that scatter the incoming light.
The use of light diffusion may be advantageous to reduce the influence of
tolerances in certain designs of the touch system. For example, the
diffusing structures may be designed to modify the energy distribution of
the incoming light, e.g. to generate a desired energy distribution of the
footprint (F2 in FIG. 2) so as to allow overlaps between successive
reflections, as discussed above. By incorporating the diffusing
structures into the facets 23 of the incoupling element 20, the diffusing
structures are arranged in the immediate vicinity of the panel 1, which
enables a well-controlled diffusion of the light within the panel 1.

[0067] The diffusing structures may be implemented as a curvature of the
individual facet 23, whereby the curvature defines a micro-lens on the
facet 23. The curvature may be in the transverse direction of the facet
23, e.g. such that the facet surface forms part of the surface area of a
cylinder. Such a diffusing structure may be used to modify the energy
distribution of the footprint. It is also conceivable that the curvature
is in the longitudinal direction of the facet 23, or in both directions
to form a spherical surface. In a variant, the diffusing structures may
be designed to generate the light diffusion (or part thereof) by
diffraction of the incoming light. Irrespective of implementation, the
coupling element 20 may be designed to produce an amount of diffusion in
the order of a few degrees.

[0068] In another example, the facets 23 (or a subset thereof) incorporate
beam separating structures. FIG. 9A is a perspective view of such an
incoupling element 20, in which an array of non-transmissive, essentially
identical, parallel, elongated lines 90 are provided in the longitudinal
direction of the facets 23. As shown, the lines 90 essentially extend in
the transverse direction of the facets 23. The lines 90 may, e.g., be
formed by depositions of non-transmissive material, or be formed as
grooves in the facet surface. In one embodiment, the lines 90
collectively define a transmission grating that separates the transmitted
light into a number of beams with different and well-defined scan angles.
FIG. 9B is a plan view to illustrate the use of such a diffractive
incoupling element 20. An incoming beam of light is swept in direction R2
along the incoupling element 20, which couples the light into the panel
for propagation by TIR while generating a zero-order beam B2 as well as
first-order beams B1, B3 on the sides of the zero-order beam. Although
not shown on the drawing, the incoupling element 20 may be designed to
generate beams of higher orders as well. It is realized that this is a
compact, robust and cost-effective way of generating plural light sheets
with well-defined scan angles in the panel. In a variant (not shown), the
beam separating structures are implemented as planar "sub-facets" of
different inclination that are arranged in the longitudinal direction of
the facet 23. Each inclination results in a dedicated refraction of the
incoming light, thereby resulting in a number of beams of different
directions. Reverting to FIG. 9A, reference numerals 90A and 90C may
represent sub-facets with a small inclination to the left and right with
respect to the incoming light, and reference numeral 90B may represent
sub-facets that are perpendicular to the incoming light. Thereby, with
reference to FIG. 9B, sub-facets 90A, 90B and 90C may generate beams B3,
B2 and B1, respectively. In yet another variant, the beam separating
structures are implemented by a combination of refractive and diffractive
structures.

[0069] The invention has mainly been described above with reference to a
few embodiments. However, as is readily appreciated by a person skilled
in the art, other embodiments than the ones disclosed above are equally
possible within the scope and spirit of the invention, which is defined
and limited only by the appended patent claims.

[0070] The touch system illustrated and discussed in the foregoing is
merely given as an example. The inventive incoupling structure is useful
in any touch system that operates by transmitting light inside a light
transmissive panel. As noted in the Background section, such touch
systems may be configured to detect the light that is frustrated and then
scattered at the point of touch, or to detect the energy decrease in the
propagating light caused by the frustration and scattering at the point
of touch. In addition to the examples given in the Background section,
some further examples of touch systems may be found in U.S. Pat. No.
7,432,893, US2006/0114237, US2007/0075648, WO2009/048365, WO2010/006883,
WO2010/006884, WO2010/006885, WO2010/006886, WO2010/064983 and
WO2008/038066, which are all incorporated herein by this reference.